This invention relates to a novel CRT (cathode-ray tube) and particularly to a CRT having an internal electrically-conductive stripe or patch and to a method for preparing that stripe or patch in a CRT.
A CRT usually includes an evacuated glass envelope comprising a faceplate panel and a funnel. An electrically-conductive funnel coating or layer is supported on the inside wall of the funnel, and a metal anode button is sealed into and through the wall of the funnel and is in electrical contact with the funnel coating. The funnel coating is maintained at a high electrical potential that is applied at the anode button. A metallized luminescent viewing screen is supported on the inner surface of the faceplate panel of the envelope. Some CRT types include an apertured-mask assembly closely spaced from the viewing screen and supported on several metal studs imbedded in the sidewall of the panel. The metallization, which is an electrically-conductive layer, usually extends over the inner panel sidewall close to the studs. The mask assembly and the metallization of the viewing screen are also maintained at the high electrical potential.
The proper operation of the CRT requires that there be electrical continuity between the mask assembly, the metallization of the viewing screen and the anode button. When there is a discontinuity, electrical charges build up on the mask and/or the screen, causing erratic operation of the CRT. Usually the break in continuity occurs on the inside surface of the envelope adjacent the studs or the anode button, which are conductive metal bodies that are attached to the envelope. In order to assure electrical continuity, it has been the practice to apply an electrically-conductive coating in the form of a stripe or patch over a portion of at least one of the studs and the adjacent metallization, and/or over a portion of the anode button and over or under the adjacent funnel coating. This has been done by brush painting or otherwise coating a stripe or patch of a composition which, after drying and heating at about 400.degree. C., forms a chemically-stable, electrically-conductive stripe or patch across the area of interest. Most prior stripes or patches consist essentially of graphite with an alkali silicate binder, similar in character to the funnel coating. Such prior coating compositions have several disadvantages for these applications. For example, they are relatively slow to dry; and, after they dry, they are not sufficiently insoluble in water to withstand subsequent processing with aqueous media. Thus, they are not adapted to be applied to the panel before the metallization is produced on the screen.